Modern integrated circuit manufacturing processes occasionally require the formation of two layers or structures of similar material and the subsequent removal of one of these layers or structures. For example, in various circumstances it may be desirable to form an initial silicon dioxide layer and subsequently form a second silicon dioxide layer upon the first layer. Later it may be desirable to preferentially etch the second layer without risk of substantial damage to the first layer.
Under other circumstances, it may be desirable to form two spacers, each spacer being made from generally silicon dioxide, and then etch (often in a wet solution) one of the spacers away. The spacer which is etched away is often termed a "disposable spacer."
Often spacers are formed by chemical vapor deposition processes in which a precursor gas is decomposed to produce the desired material layer.
In each of the above circumstances, it is desired to form two layers, generally composed of silicon dioxide, one of the layers having a high wet etch rate, good step coverage (in spacer applications) and acceptable formation rates in a production environment. The other layer should desirably have a lower wet etch rate.